The present invention relates to an apparatus for and a method of ladle refining which is a secondary refining process of molten steel.
Quality demands for steel products have lately become more and more stringent as steel application technologies have advanced and diversified and, as a consequence, needs for production of high purity steels have further increased. In response to the needs for high purity steel production, many apparatuses for hot metal pretreatment and secondary refining have been newly constructed at steelmaking plants. As apparatuses for secondary refining in particular, vacuum refining apparatuses such as RH, DH, and the like, and electric arc-heating slag refining apparatuses typically such as LF are commonly used for purposes of degassing and inclusion removal of molten steel. For producing high purity steels such as bearing steels, treatment processes combining the LF, RH, and the like, as needed, are also commonly practiced.
However, there is a certain limit to refining capacity for removing inclusions in case of an apparatus such as an RH vacuum refining apparatus, wherein a vacuum refining treatment is performed by inserting an immersion tube into molten steel in a ladle and sucking up the molten steel into a vacuum chamber through the immersion tube, because: slag is not sufficiently reformed since a force to agitate the molten steel in the ladle is weak and the slag on a molten steel surface outside the immersion tube is not sufficiently agitated, and, as a result, the molten steel is re-oxidized by highly oxidizing slag; and the molten steel is re-oxidized also by a reaction between iron oxides contained in skulls deposited in the vacuum chamber and the molten steel in the vacuum chamber. Methods to lower an oxidizing capacity of slag by a combined use of an LF apparatus and the like are commonly practiced for the purpose of preventing steel re-oxidatipn by the slag and consequent deterioration of purity of molten steel. But these methods have problems of protracted processing time and increased production costs caused by greater heat loss, wear of refractories, etc. resulting from a long processing time.
In view of these problems, as measures of conventional technologies to effectively accelerate reaction between slag and molten steel under a vacuum by directly reducing atmospheric pressure on the molten steel surface in a ladle, a VOD process, a VAD process, an SS-VOD process and the like have been developed. As methods to directly reduce atmospheric pressure on the molten steel surface in a ladle, there are a method to expose the ladle in its entirety to a reduced atmospheric pressure by placing the ladle inside a decompression vessel capable of accommodating the entire ladle, and another method to reduce atmospheric pressure on the molten steel surface in a ladle by using the ladle itself as a lower decompression chamber and having an upper decompression chamber tightly coupled to a top of the ladle. Both methods have problems in that the equipment is complicated and that, owing to their structural restrictions, it is impossible to inject a great amount of agitation gas for preventing molten steel or slag from splashing. Thus, these methods have not been widely used for reasons of productivity, equipment cost and maintenance.
From the above viewpoints, as an invention to improve a method of exposing an entire ladle to a reduced atmospheric pressure by placing the ladle inside a vacuum/decompression vessel capable of accommodating the entire ladle, Japanese Unexamined Patent Publication No. H9-111331 discloses a method capable of coping with molten steel splashing and slag foaming during vacuum processing and reducing a processing time by installing an inner tube having a sufficiently large free board inside a vacuum chamber. However, this method is a method to refine steel by placing an entire ladle inside a vacuum chamber which is divided into upper and lower sections and whose inner diameter is larger than an outer diameter of a top end of the ladle, and uses facilities such configured so that a lower end of the inner tube is tightly coupled to a top end of the ladle, or is immersed in slag and molten steel in the ladle. For this reason, it is feared that, during vacuum refining, attachment and/or detachment of the inner tube to/from the ladle may become impossible owing to skulls caused by splashes of the molten steel, or the molten steel is contaminated by the skulls in the case that the inner tube is immersed in the molten steel in the ladle. Further, this method has another problem of difficulty in securing a temperature of molten steel when processing time is prolonged.
As a method to reduce atmospheric pressure on a molten steel surface in a ladle by using the ladle itself as a lower decompression chamber, and tightly coupling an upper decompression chamber to atop of the ladle, disclosed in Current Advances in Materials and Processes, Vol. 3, No. 1, 1990, p250 (published by the Iron and Steel Institute of Japan) is a method to prevent splashes generated on the molten steel surface by gas injected through a ladle bottom from directly contacting a coupling portion (ladle sealing portion) between the ladle and the upper decompression chamber by installing an inner lid at an upper part of the ladle, and to prevent splashes from flying over the inner lid and contacting the ladle sealing portion by installing a shielding board at the upper part of the ladle. This method, however, has problems in that the attachment and/or detachment of the inner lid may be rendered impossible by skulls formed by the molten steel splashes and that a refractory cost of the shielding board itself becomes significant since the molten steel splashes also adhere to it. Further, there is another problem in that workability is poor because the inner lid and the shielding board have to be attached and detached at every vacuum treatment cycle.
The present invention provides a ladle refining apparatus capable of easily solving the problems of the conventional technologies, and a ladle refining method using the apparatus. The present invention is, namely, an apparatus for and a method of ladle refining capable of efficiently producing high purity steels and significantly improving thermal tolerance by: radically improving operational difficulties and contamination of molten steel, which have constituted the problems of conventional ladle refining methods, through suppressing skull deposition caused by splashing of molten steel; and, at the same time, performing molten steel agitation, slag reforming and degassing efficiently.
The present invention is a vacuum/decompression refining apparatus to refine molten steel in a ladle by directly coupling a vacuum/decompression chamber 2, not having at its lower end an immersion tube to be immersed into molten steel 4 in a ladle 1, to an upper part of the ladle and reducing internal pressure of the vacuum/decompression chamber, and by agitating the molten steel in the ladle through injection of inert gas into the ladle. The apparatus is characterized in that: the upper part of the ladle is tightly coupled to the vacuum/decompression chamber to form a sealed structure; the vacuum/decompression chamber has a shaft portion; an inner diameter of the shaft portion is smaller than an inner diameter of a top end of the ladle but not smaller than the projected diameter of a bulging portion 7 of the molten steel surface in the ladle formed by agitation gas injected into the ladle; and a height from a surface of the molten steel in the ladle to a top of the vacuum/decompression chamber is 5 m or more.
Further, the present invention is a vacuum/decompression apparatus characterized in that: a lower end of vacuum/decompression chamber 2 is provided with a cylindrical appendage 9. The cylindrical appendage has an inner diameter equal to or larger than a projected diameter of a bulging portion of molten steel in ladle 1, and an outer diameter equal to or smaller than the inner diameter of the top end of the ladle. Also, a lower end of the cylindrical appendage extends lower than a top of the ladle but is not immersed in the molten steel in the ladle.
Furthermore, the present invention is a vacuum/decompression apparatus capable of heating molten steel 4 and maintaining a temperature in a vacuum/decompression chamber by installing a burner 10, which discharges flame from its lower end, by burning fuel and oxygen gas, inside vacuum/decompression chamber 2. The present invention is, further, a steel refining method using the aforementioned vacuum/decompression apparatus, characterized by constantly maintaining a temperature of an inner wall of the vacuum/decompression chamber at 1,000xc2x0 C. or higher, during continuous operations, by the flame discharged from the lower end of the heating burner 10.
Yet further, the present invention is a ladle refining method characterized by refining molten steel, when using the vacuum refining apparatus, in a manner that an amount of slag on a surface of the molten steel in the ladle satisfies the following expression:
0.010xe2x89xa6H/hxe2x89xa60.025,
where, H is a thickness of the slag in the ladle and h is a depth of a molten steel bath in the ladle.
In addition, the present invention is a ladle refining method characterized by controlling pressure in a vacuum/decompression chamber to 760 to 500 Torr when heating molten steel by adding Al to molten steel and burning the added Al by supplying oxygen gas.